Save as format “Excel 97–2003”

This document may contain formatting or content that cannot be saved in the currently selected file format “Excel 97–2003”

How can I find out what content that is?


not at all - this is a general message which appears in any case you store a file in a non-ODF format.

If not somebody remembers precisely related experiences, and reports here, I see only one way to find out something about the differences, and mainly if they are relevant for you:
Save the document to .ods and to an interim file despite the warning, open that alien-style file again with your LibreOffice, and (if you have access to) with MS Excel 2003 through any current versions (MS made it complicated).

I would expect the relevant functionality untouched if not you used special tools like the PivotTable (DataPilot) that may not fully map to MS concepts.
I also would expect that not all of the formatting will be preserved.
And thirdly I am sure you cannot use the result if the original document’s functioning depended on user code (“macros”).

Finally I feel sure that nobody will know a reliably complete answer. Please consider that the term “file format” is misleading to a certain extent: Spreadsheet software is based on concepts about what a spreadsheet should be, how it should work, and how its internal objects should “cooperate”. Different brands will partly follow different concepts - and each alien concept may simply not be “mappable” in a specific “file format”. In addition Excel 1997-2003 has a proprietary binary format (not documented to the public until 2007). Developers of the late StarOffice on wich (LibO is founded) needed to “reverse engineer” it. Also consider that incompatibility is part of the struggle for life (and earnings) for a commercial software competeing with a free one.

Beyond “finally”: Reconsider your reasons to save to .xls (1997-2003). If your intention is to have the document in a file different memebers of a group will edit with different software again and again, I would suggest to cancel the idea. Its proven to cause errors.